1. Field of the Invention
The present invention relates to a webbing take-up device equipped with locking means which prevents rotation of a winding shaft in a direction in which a webbing is pulled out at the time of sudden deceleration of a vehicle.
2. Description of the Related Art
FIG. 9 schematically shows a locking structure of a conventional webbing take-up device 150. As illustrated in this figure, the webbing take-up device 150 includes a frame 152 and the frame 152 is fixed to the side of a vehicle body. An internal gear ratchet 156 is formed in a leg plate 154 which is one of leg plates provided in the frame 152. At the side of an inner periphery of the internal gear ratchet 156, a pair of lock plates 158 and 160 each having a substantially crescent-shaped plate are disposed in such a manner as to be substantially point-symmetrical about the center of the internal gear ratchet 156. Locking teeth 158A and 160A are formed respectively in portions of outer peripheries of the lock plates 158 and 160 and face a ratchet gear 156A of the internal gear ratchet 156.
Further, engagement concave portions 162 and 164 are formed respectively in intermediate portions of the pair of lock plates 158 and 160, and a branched portion 166A of a winding shaft 166 projecting from an axial core portion of the internal gear ratchet 156 is disposed in the engagement concave portions 162 and 164.
Moreover, a V gear 168 rotating by following the winding shaft 166 is provided at a position adjacent to the internal gear ratchet 156. Lock teeth 168A are formed in the outer periphery of the V gear 168, and four guide holes 170 and 172 are formed substantially at ends of an axial core portion of the V gear 168. A pair of projections 158B projecting from the lock plate 158 and a pair of projections 160B projecting from the lock plate 160 are inserted respectively in the guide holes 170 and the guide holes 172.
According to the above-described structure, when sudden deceleration of a vehicle is detected by an acceleration sensor (not shown), a pawl of a sensor lever of the acceleration sensor is engaged with the lock tooth 168A of the V gear 168 to thereby prevent rotation of the V gear 168 in a direction in which a webbing is pulled out. For this reason, relative rotation is caused between the winding shaft 166 which is about to rotate in the direction in which the webbing is pulled out due to webbing tension generated by inertia movement of a vehicle occupant, and the V gear 168 which is prevented from rotating in the direction in which the webbing is pulled out. For this reason, the projections 158B and 160B are guided by the guide holes 170 and 172 of the V gear 168 (so as to respectively move in the guide holes 170 and 172), and the pair of lock plates 158 and 160 are moved in directions apart from each other to allow the lock teeth 158A and 160A to be engaged with the ratchet gear 156A of the internal gear ratchet 156. As a result, as illustrated in FIG. 9, the rotation of the winding shaft 166 in the direction in which the webbing is pulled out is prevented and the webbing take-up device is thereby placed in a locked state.
The above-described locking structure conventionally used widely is provided to maintain lock strength by using two lock plates 158 and 160. However, the number of parts increases for the reason that the two lock plates 158 and 160 are used, and an increase in the weight of the webbing take-up device 150 is thereby caused.